The amino acid sequence of the first 40-50 residues of the N-terminal portion of murine H-2K, H-2D, TL and F9 (one product of a T/t allele) will be defined. These are 44 kilodalton gene products of chromosome 17 and each is expressed on the surface of cells. All appear to function in some manner as recognition units. The major approach set forth in this proposal will be to label the appropriate cells with one H3-amino acid at a time, under conditions in which there is minimal metabolic interconversion into other amino acids. The 44 kilodalton subunit will be isolated by immunoprecipitation and acrylamide gel electrophoresis in sodium-dodecyl sulfate. The peak will be isolated, eluted, small molecules removed, aliquots examined in the amino acid analyzer to prove lack of interconversion, and the remainder of the eluate subjected to automatic sequencing on the Beckman Sequenator. The position of the label will establish the position(s) of the single amino acid used. This approach has already been used successfully to establish the positions of 4 amino acids in the first 40 residues of H-2K. A second approach will be to label uniformly and use a high pressure liquid chromatography analysis in conjunction with the sequenator to determine which amino acid is at a particular position. When fully developed, this latter technique would enormously shorten the time needed to sequence. These approaches may be extended to the study of additional molecules such as Ia and to analyzing residues "deeper" in the molecule (using selective chemical and/or enzymatic digestion and separation of radioactive peptides). The results will be important in helping to elucidate structural relationships among products of the 17th chromosome and between these products and other known recognition molecules such as the immunoglobulins.